The Automatic Identification System (AIS) is a maritime communications system designed for short-range (typically 20-30 nautical miles) ship-to-ship and ship-to-shore communications. The AIS system uses narrowband (i.e. 25 kHz Bandwidth) Very High Frequency (VHF) channels centered at 161.975 MHz and 162.025 MHz, with a possible additional channel at 157.375 MHz, and a communication method called Self-Organizing Time Division Multiple Access (SOTDMA).
The AIS system supports a number of different types of signal. The principal AIS signal sent by a ship is a position report that provides information pertaining to the ship's identification, location, course, speed, and other details. A position report signal typically includes a ramp-up field, a training sequence, a start flag, a message segment field, a Frame Check Sequence (FCS), an end flag field, and a buffer field. It is the message segment field that contains information pertaining to the ship from which the AIS signal was sent, such as the ship MMSI (Maritime Mobile Service Identity) and the longitude and latitude of the ship's location. The message segment field may also include other information related to the ship including the navigation status, rate of turn, true heading, etc., as well as other information such as a time stamp indicating when the information was obtained.
The AIS system also includes the use of a receiver, enabling a ship to receive AIS signals emitted by ships around it and extract the message segments contained therein. Each minute, each VHF channel is divided into 2,250 time slots, each of which can accommodate one 26.67 ms AIS transmission (i.e. AIS signal). The time slots are accurately synchronized to Coordinated Universal Time (UTC) typically using Global Positioning System (GPS), and each AIS unit reserves time slots for future AIS transmissions from the ship. Other AIS units within range can therefore maintain a map of reserved slots and avoid transmitting during these intervals. This self-organizing feature avoids signal collisions over the short ranges involved in surface transmissions.
The use of AIS is now mandatory on all ships over 300 tons engaged on international voyages, and it is also being extended to other vessels. It was originally conceived as an aid to navigation and safety, and also has potential security applications for monitoring maritime traffic. AIS signal detection could be achieved using coastal/ground stations, but the limited range of the VHF signals would require such coastal/ground stations to be situated at many locations along the coast, and even then they could only monitor the immediate coastal region.